Mounting evidence implicates glutamate and the process of excitotoxicity in the pathogenesis of ALS. In this study, we are looking at the effect of Dextromethorphan - a selective antagonist of the NMDA subtype of the glutamate receptor-on ALS. This is a phase II, double blind, placebo controlled study to be carried out over a three year period with a target enrollment of 60 patients.